A movable image combiner of the type described above is usually located in the inactive position when the vehicle is parked, and moves into the active position when the vehicle is started or in response to a dedicated driver request. A known head-up display device is taught by DE 10 2004 015 275 A1, wherein the image combiner is formed by a prism, which can be lowered in the instrument panel. The present invention, however, is directed to all possible head-up displays having a separate image combiner, e.g., having a simple disk made from glass or plastic, which can be retracted into the instrument panel or can be folded down to be flush (or nearly flush) with a surface thereof In addition, the term “image combiner”, as used here, is intended to mean not only devices having a projection area, onto which the information from the instrument panel that is supposed to be displayed to the driver is projected and, there, is combined with the scenery that can be seen in the direction of travel, but also any other transparent displays having light pixels integrated therein and that have already been realized and that can provide the driver with a view all the way through.
In the case of such head-up displays having a separate image combiner, it is generally preferable to situate the image combiner as close as possible to the windshield, inter alia, because the virtual image of the information displayed to the driver is supposed to be located as far away from the eyes of the driver as possible, so that the eyes of the driver do not need to focus back and forth too extremely when moving between the scenery in front of the vehicle and the displayed information.
An image combiner located close to the windshield, due simply to its distance from the eyes of the driver, should also be relatively wide and may extend up close to the driver-side A-pillar. In addition, separate image combiners are preferably used in vehicles having sharply-angled (or raked) windshields, which devices are unsuitable for direct projection onto the windshield, and, in this case, the image combiner should stand substantially more upright than the windshield. In this case, an intermediate space between the windshield and the image combiner results, the space having an approximately triangular cross section. Even if recirculation flaps or defroster nozzles on the instrument panel are advantageously disposed in the space between the windshield and the image combiner, regions form—as the inventors have determined—having very little air flow on the windshield, in which areas the windshield fogs up particularly easily and, in fact, first in its lower corner toward the A-pillar.
The possibility that a windshield having an image combiner located closely thereto may tend to fog up is known per se from JP 2008213719 A, and said document addresses the problem by designing the image combiner with an integrated air distribution duct, through which defroster air is blown, which air impacts the windshield at the upper end of the image combiner.